Enzymes in phosphorous cycling in the coral holobiont

A review of: Thomas S, Burdett H, Temperton B, Wick R, Snelling D, McGrath JW, Quinn JP, Munn C, Gilbert JA (2010) Evidence for phosphonate usage in the coral holobiont. International Society for Microbial Ecology Journal, 4: 459-461.

Reef systems are oligotrophic environments where efficient nutrient recycling is absolutely essential. Phosphonates which contain thermally and hydrolytically stable carbon-phosphate bonds, account for 25% of the dissolved organic phosphorus pool in the marine environment. The cleavage of the carbon-phosphate bond is facilitated by a series of substrate specific phosphonohydrolases, which free up phosphate groups for subsequent utilization by microorganisms that form part of the coral holobiont. Phosphonates are widely distributed in the phylum Cnidaria as membrane lipid and protein conjugates, therefore the presence of a microbe facilitated phosphonohydrolase-catalysed phosphonate production was proposed by the authors. The phnA gene encoding the enzyme phosphonoacetate hydrolase, which hydrolyses phosphonoacetate into phosphate and acetate, has previously been identified in open water samples.

DNA was extracted from 13 tropical and temperate coral holobionts from the National Marine Aquarium (Plymouth) and PCR amplified using degenerate primers. Eight phnA homologues were subsequently identified. Clone libraries were prepared from the phnA PCR products from two tropical soft coral species (Discosoma sp. and Sinularia sp.) and a total of 256 phnA sequences were characterized. It was determined that the phnA homologues from both species were considerably more diverse than those described in the open ocean. Moreover, the phnA homologues were found to be specific to bacterial species, corals and coastal waters. To identify the bacterial species that contained the phnA homologues, Sinularia sp. tissue samples were plated on marine agar containing phosphonoacetate as the only source of carbon and phosphate. Sixteen bacterial taxa were identified using 16S rDNA sequencing but only eight had a phnA homologue. Seven of the eight genes were deemed identical and comprised one group while the one from Alteromonas was considered a separate group. The phnA homologues similarity suggests the gene is highly conserved or may be a result of horizontal gene transfer.

All sequenced Vibrios in the NCBI database have the phosphonatase pathway, involving phnW (2-AEP transaminase) and phnX (phosphonoacetaldehyde hydrolase) genes. phnX was identified in all the Vibrios isolated in the investigation, suggesting that potential coral pathogens have multiple phosphonate degradation pathways. In addition, the phosphonoacetaldehyde produced from transamination by AEP transaminase is believed to be converted to phosphonoacetate by an unidentified enzyme, and may possibly involve the phnA gene in the phosphonatase pathway.

The study was the first direct identification of phnA homologues from a coral holobiont, and the phosphonatase pathway may be prevalent in commensal and pathogenic bacteria. The diversity of phnA homologues in coral associations compared to pelagic waters suggests the enzyme plays a major role in symbiosis/commensalism with marine bacteria. On-going work is being conducted to determine whether the phnA gene in the coral holobiont is a key enzyme in phosphorous recycling within reef systems, and whether the gene is important in the pathogenicity of microorganisms associated with coral diseases. It would also be interesting to investigate whether the enzyme is utilised by zooxanthellae in hospite as part of the endosymbiotic relationship, and whether deleting the gene or inhibiting the enzyme in pathogenic bacteria is a viable control measure.